More than 50,000 devices had already been made and many delivered to shops by the time it was decided the deal with TomTom ran against policy which prevents the show's presenters endorsing motoring products.

The devices were still sold but BBC Worldwide - the corporation's commercial arm, which struck the deal - didn't make any profit, with proceeds going instead to Children In Need.

The new report by the BBC Trust's editorial standards committee (ESC) said BBC bosses discovered the deal after an article was published on September 4 and Mr Thompson pulled the plug less than four weeks later on September 29.

It said BBC Worldwide "failed to recognise" the potential conflict of interest and said there should have been "an immediate referral to BBC editorial policy".

The report found the breach of editorial guidelines was not "deliberate", but it could have "undermined" public trust in the BBC.

A review of the case by the BBC executive included in the report stated: "On one level the compliance process worked i.e. the editor-in-chief, as the ultimate arbiter, intervened so that the deal did not progress as proposed, but this could have been resolved much earlier if various referrals or different judgments had been made."

The motoring show is described in the report as "one of BBC Worldwide's most successful global brands" which is shown in almost 200 different countries and is backed by sales of books and other products.